INTERACTION OF 3 µm RADIATION WITH MATTER

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INTERACTION OF 3 µm RADIATION WITH MATTER

J. Frauchiger, W. Lüthy

To cite this version:

J. Frauchiger, W. Lüthy. INTERACTION OF 3 µm RADIATION WITH MATTER. Journal de

Physique Colloques, 1987, 48 (C7), pp.C7-97-C7-100. �10.1051/jphyscol:1987715�. �jpa-00226971�

INTERACTION O F 3 pm R A D I A T I O N WITH MATTER

J. FRAUCHIGER and W. LUTHY

Tnstitute of Applied Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland

Summary

Drilling of small holes into media containing OH or CH bonds with a 2.92 um YA10,:Er laser is studied. In materials containing OH bonds strong absorption allows to heat small volumina with only marginal heat diffusion. Drilling with nearly diffraction limited lateral resolution is achieved.

Introduction

Interaction of laser radiation with matter has been studied for a long time. Today, lasers are used not only in industry for applications such . a s drilling, cutting, or welding but also for medical applications like ophthalmology and surgery [ll. In surgery excimer lasers, Nd:YAG lasers or CO,-lasers are commonly used. For materials containing water or OH groups, a dominant constituent of living tissue, the lasers can still be improved by choosing a wavelength that fits the absorption maximum of water [ 2 1 . For precise drilling or cutting it is necessary to absorb the laser energy in a volume that is a s small as possible in order to minimize thermal damage in neighboring tissue. Si.nce at 3 um the spread of the beam by scattering is only marginal the irradiated volume is proportional to the penetration depth times the spot size. These parameters depend on the focusing optics and the laser wavelength.

If the heat conduction of the target material is low and the pulse duration short enough, it is possible to obtain high temperatures within the irradiated volume with minimum energy loss and heat diffusion into the surrounding material. Most of the irradiated energy is then dissipated in the removed material.

The absorption spectra of water and blood plasma and for comparison also for "Perspex" are shown in Fig. 1.

Fig. 1: Optical density A = -lOiog T (T=transmission of a 1 cn layer) and peneration depth (at l/e) of light in the wavelength range from 0.9 to 3.2 um in distilled water H z O , blood plasma BP and P?l?lA together with the emission wavelength at 2.92 vn of the YAlO,: Er laser.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987715

C7-98 JOURNAL DE PHYSIQUE

It is seen, that a very high absorption in water is found either in the mid infrared at 2.93 vm 131 or in the UV-region where an equi- valent value of the absorption is found at wavelengths below 180 nm i21. The 2.92 vm wavelength has several advantages compared to the VUV radiation. (i) 2.92 vm radiation is not absorbed In air and no special optical components are used and (ii) for 2.92 um low loss waveguides exist t41. Therefore a laser source in the 3 vm range t51 is preferred.

In our experiments we investigated the possibilities of drilling holes in materials with different penetration depths for 3 vm radiation. Table 1 lists the relevant properties [61 of the investigated target materials.

I

I

I

I

⁽

Table 1: Properties of the investigated materials with melting-point MP (values denoted with * indicate temperatures of decomposition), heat capacity c , heat conductivity 2 , density Y and penetration depth d.

Teflon [61 Perspex t61 CuS04-5(H20)[81 Hair

Experimental arrangement

In the experiments a YA103:Er laser with 50% Er doping was used. At this dopant level the laser emits only one line at 2.92 vm close to the maximum IR absorption in H20. The laser rod was pumped with a linear Xenon flashlamp with typical pump energies of 100 J. The laser pulses were monitored with an InAs photodiode for the measurement of pulse shape and with an energy meter. For precision processing the typical output energy of 20 mJ was reduced by neutral density filters. F/5 optics yeald for a Gaussian beam a minimum focus diameter of 5 urn. In our experiments we achieved 15 vm (see Fig 3 b )

C-F C-H 0-H 0-H, C-H

Experiments

In a first experiment materials with a large penetration depth in the order of 1 mm such as "Teflon" and "Perspex" were irradiated. In

"Teflon" no remaining change in surface morphology was found after laser irradiation with energies up to 1 mJ. This is due to the properties of "Teflon" given in Tab. 1 , especially to the penetration depth of 1.6 mm and the melting temperature of 325 OC.

It should be noted, that with respect to the visible wavelength range at 2.92 scattering is strongly reduced. In "Perspex" the penetration depth is 510 vm. The drilling mechanism is based on boilingof the material or its fragments with expulsion of liquid and gaseous material. The drilling process is very much the same as it was observed in silicon t71.

In a second experiment we irradiated inorganic material containing crystal water. In the case of CuSOa(-5H20) the density of crystal water is 0.83 g/cm3. Therefore we assume, that the absorption at

t0Cl

325 120*

11 0 *

J

t- I

"C

-

1054 1465

Fig. 2a) Shape of holes drilled with 750 uJ, 400 uJ, 113 uJ and 32 yJ respectively.

a)

Expulsion of matter was found to start already at a laser energy of about 1 uJ. Figure 2a) shows the characteristic shape of holes drilled with different pulse energies, as obtained by microscopic inspection of cleaved samples. Figure 2b) shows a plot of the expulsed mass as a function of laser energy. The four measured values are denoted by + and the solid line is a linear approximation. Its slope gives the efficiency of the drilling process (0.32 mg/J). Similar results have been obtained in CaS04-(2H,O).

Assuming the same efficiency in living tissue, a cut of 20um wldth and a depth of 1 mm might be performed at a speed of 1 cm/s with a 1 W Erbium laser.

t

/-+'zq - -

In a third experiment biological material was irradiated to de- monstrate the lateral resolution of the drilling process in complex structures and the thickness of the thermally damaged wall material.

Human hair was used as target material. As shown in Fig. 3 irradiation with pulse energies of a ) 90 UJ produces holes with about 25 ym diameter of 30um depth and b) with 2 pulses of 90 UJ energy each the hair is shot through.

i ^-

C + / + I I

Fig. 3: Holes in human hair, produced by single laser pulses:

a) Irradiation with 90 uJ energy each. The depth of the holes is about 30 ym. b) A gold wire of 12 um diameter is passed through a hair, drilled through by two laser pulses of 90 UJ energy each.

1- 0

E

C

0 250

ELI]

a b

C7-100 JOURNAL DE PHYSIQUE

In conclusion we have shown that the strong absorption at 2.92 vm in H20 leads to heated volumina, so small, that a laser energy of few uJ leads to evaporation. Even with relatively long laser pulses of 80 us only marginal heat diffusion occurs. In interaction with organic material containing H20 a lateral resolution close to the diffraction limit can be achieved.

Acknowledgments

We thank H.P. Weber for helpful discussions and P.Herren and M. Frey for their support.This work was supported in part by the Swiss Commission for the Encouragement of Scientific Research.

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